In general, electromagnetic valves (solenoid valves) are equipped with a coil, a fixed core, and a movable core and are configured as follows. The movable core is excited by generating a magnetic field by a current flowing through the coil, whereby the movable core and the fixed core attract each other and the valve is opened or closed by this attraction force.
Normally open electromagnetic valves are equipped with a return spring to open the valve by moving a valve member away from a valve seat surface in an ordinary operation state. The valve member is pressed against the valve seat surface when the coil is energized.
Incidentally, in linear solenoid valves in which the produced differential pressure can be varied according to the value of current flowing through the coil, the valve member is slightly separated from the valve seat surface in a differential pressure control state. If a ripple occurs in the fluid pressure in this state, the valve member may be rendered unstable being affected by such a variation in the fluid pressure. In view of the above, JP-B-3,008,240, JP-UM-A-60-24983, and JP-A-2008-196642 disclose structures for stabilizing the movable core and the valve member when the movable core has moved to the valve closing side by causing magnetic force to act between the movable core and the fixed core in the radial direction. More specifically, the movable core is provided with a projection which projects toward the fixed core and the fixed core is provided with a recess which corresponds to the projection in diameter in such a manner that the projection goes into the recess and is opposed to the recess in the radial direction when the valve is closed (i.e., during a differential pressure control).
However, in the structures disclosed in the above documents, the fixed core needs to be formed, by mechanical processing, with the recess to receive the projection of the movable core separately from a cylindrical sliding contact surface for supporting the valve member that is advancing or retreating. This processing causes an additional cost. Since the recess is larger in diameter than the sliding surface, the area of the end face, opposed to the movable core, of the fixed core is reduced. This results in a problem that a large current is necessary to produce strong thrust (valve closing force) in the advance/retreat direction of the movable core.